This invention relates to an apparatus and a method for cooling a wafer-like substrate in a substrate treating system. More particularly, the invention relates to cooling of a substrate by convection in a location not in the direct path of the treating system.
Semiconductor wafers or other such substrates are subjected to very high processing temperatures. For example, in chemical vapor deposition (CVD), the temperatures approach 1200xc2x0 C. In a typical cycle, a wafer is transferred from a room temperature cassette by a robotic wafer handler into a reactor chamber where it is subjected to the high temperature processing and is then transferred by the wafer handler from the high temperature chamber back to the same cassette or a separate cassette for processed wafers. Because of the high temperature CVD processing, transport from the process chamber directly to a wafer cassette is not possible due to the temperature of the wafer exceeding the material properties of most commonly used cassette materials. Because of this, the transfer of the wafer to a cassette must be postponed until the wafer temperature falls below the thermal properties of the cassette material. While cassettes are available that can handle wafers as hot as 170xc2x0 C., they are relatively expensive. A commonly available less expensive one made of Delrin7 can only handle temperatures well below 100xc2x0 C. Other commonly available units can only handle about 60xc2x0 C. Hence, it is a desirable goal that the temperature of a wafer be quickly cooled to that level.
Because the wafer handling and processing occurs in an enclosed and carefully controlled environment, there are essentially only three locations or points during the cycle where the cooling of the wafer might occur. The wafer could be cooled on the susceptor on which it is supported in the process chamber, on the wafer handling device, or off-line at some location within the apparatus. Cooling the wafer on the susceptor is not cost-effective because the process chamber is then unavailable for processing another wafer, thereby reducing the system wafer throughput. This approach is particularly unattractive because it is then necessary to incur the delay and cost of reheating the chamber. Removing a wafer while it is hot and cooling it on the wafer handling device is better, but also not cost effective because the delay in loading the next wafer slated for processing compromises throughput. Such impediments increase the per-wafer cost, making these approaches financially unattractive to end users. Because of the high cost of semiconductor wafer processing equipment, it is, of course, critically important from a competitive standpoint to be able to keep the expensive equipment in continued use so as to increase the throughput. At the same time, the wafer cooling technique employed must be compatible with the environment of the CVD processing apparatus so as not to adversely affect stringent cleanliness requirements. Also, the cost of the technique must itself be sufficiently moderate so that there is a net reduction in the per-wafer cost.
Accordingly, it is an object of this invention to provide an improved system for quickly cooling wafer-like substrates to a temperature that will allow the use of low cost commonly available cassettes.
In accordance with one aspect of the invention, a substitute or cooling station is provided in a wafer handling chamber located between a wafer input/output storage area and a wafer process chamber. The cooling station is located so that the wafer handling device may be utilized while a wafer is cooling to transfer another wafer from storage into a process chamber and to transfer a cooled wafer into storage while the wafer is being processed.
In a preferred form of the apparatus and method, a processed wafer is withdrawn from the process chamber at a high temperature and placed by the wafer handler into a cooling station where it is sprayed by gas which is compatible with the gases being utilized in the wafer handling chamber. Preferably, the gas is sprayed onto both flat surfaces of the wafer by an upper showerhead and by a lower showerhead. While that wafer is being cooled, the wafer handler is operated to transport a second wafer into the process chamber.
It is desirable that a second cooling station be provided on the opposite side of the wafer handling chamber from the first one so that a second wafer may be moved from the wafer storage area into the second cooling station. Thus, as soon as the first wafer is transferred from the process chamber into the first cooling station, the wafer handler may move directly to the second cooling station and move that second wafer into the process chamber. In such an arrangement, the second cooling station is, in effect, serving as a staging area so that the time for moving the second wafer into the process chamber is reduced from what it would be if the wafer handler had to move the second wafer from the wafer storage area into the process chamber. Alternatively, if it is desired to have a wafer cooling in each station at the same time, a wafer may be transported directly from the storage area to the process chamber.
In a preferred form of the invention, the wafer handler includes a Bernoulli wand, which is particularly well adapted for moving a wafer into and out of a process chamber, and further includes a paddle, particularly well adapted for moving a wafer into and out of a standard cassette for wafers. It is advantageous to be able to use the high temperature wand because it does not touch the upper surface of the wafer and it cools as it carries. Further, using the paddle for transporting a wafer between the cassette and the cooling station is desirable because it fits between wafers in a standard cassette.
Another advantage of the system of propelling gas against the surfaces of a wafer in a cooling station is that the rate of cooling may be easily controlled by the rate at which the cooling gas is projected onto the wafer. Thus, the flow rate can be adjusted to minimize the overall cycle time. The overall system provides a significant reduction in the wafer processing time and, consequently, reduces the cost per wafer. Further, this advantage is obtained without adverse effect on the clean environment required in CVD apparatus.